查询Q（）不能作为Django中相关字段的“设置包含”吗？

Question

Let's say we have baskets of fruits. How to filter out those baskets that contains all fruits in a given basket?

In this document https://docs.djangoproject.com/en/dev/ref/models/querysets/#in 'in' method seems will return any basket which contains any of the given fruit. Is there any "set_contains" method to be used for filtering ? Such as Basket.objects.filter(fruit_set_containsAll=fruitSet) ?

Edit: I found that Q() is not working as expected. I'll post the test here:

class Basket(models.Model):
    weight = models.FloatField(default=1)
class Fruitname(models.Model):
    name = models.CharField(max_length=32)
class Fruit(models.Model):
    ofkind = models.ForeignKey(Fruitname, on_delete=models.CASCADE)  
    inbasket = models.ForeignKey(Basket, on_delete=models.CASCADE)  
    weight = models.FloatField(default=1)

the database is set as 'apple' only in 1 basket, 'pear' in 2 baskets, and 'banana' in 3 basket:

print Basket.objects.all()
print Fruitname.objects.all()
[<Basket: id 31 has 4 fruits 
    1. id - 53 apple(28), weight 1.00
    2. id - 54 apple(28), weight 2.00
    3. id - 55 apple(28), weight 3.00
    4. id - 62 banana(30), weight 10.00
>, <Basket: id 32 has 2 fruits 
    1. id - 56 pear(29), weight 4.00
    2. id - 57 banana(30), weight 5.00
>, <Basket: id 33 has 4 fruits 
    1. id - 58 pear(29), weight 6.00
    2. id - 59 banana(30), weight 7.00
    3. id - 60 pear(29), weight 8.00
    4. id - 61 pear(29), weight 9.00
>]
[<Fruitname: apple(28)>, <Fruitname: pear(29)>, <Fruitname: banana(30)>]

If I try to query with 'apple' and 'banana', it gives empty set !!

print Basket.objects.filter(Q(fruit__ofkind__name__in=['apple'])&Q(fruit__ofkind__name__in=['banana'])).distinct()
[]

similarly,

print Basket.objects.filter(Q(fruit__ofkind__name__in=['banana'])&Q(fruit__ofkind__name__in=['pear'])).distinct()
[]

Here's how I use 'in' to filter, it is not what I need which is supposed to be an empty set.

print Basket.objects.filter(Q(fruit__ofkind__name__in=['apple','pear'])).distinct()
[<Basket: id 31 has 4 fruits 
    1. id - 53 apple(28), weight 1.00
    2. id - 54 apple(28), weight 2.00
    3. id - 55 apple(28), weight 3.00
    4. id - 62 banana(30), weight 10.00
>, <Basket: id 32 has 2 fruits 
    1. id - 56 pear(29), weight 4.00
    2. id - 57 banana(30), weight 5.00
>, <Basket: id 33 has 4 fruits 
    1. id - 58 pear(29), weight 6.00
    2. id - 59 banana(30), weight 7.00
    3. id - 60 pear(29), weight 8.00
    4. id - 61 pear(29), weight 9.00
>]

The only way that is working properly is chaining with filter:

Basket.objects.filter(fruit__ofkind__name__in=['apple']).filter(fruit__ofkind__name__in=['banana']).distinct()
[<Basket: id 31 has 4 fruits 
    1. id - 53 apple(28), weight 1.00
    2. id - 54 apple(28), weight 2.00
    3. id - 55 apple(28), weight 3.00
    4. id - 62 banana(30), weight 10.00
>]

These code is tested with Django 1.9.4 Any explaining? I would undo the accepted answer for the moment.

Answer 1

Not sure if this is the most performant way to go, but at least it should translate to one (big, ugly, nested) sql transaction (once the final baskets queryset is evaluated):

baskets = Basket.objects.all()
for fruit in fruits:
    baskets = baskets.filter(id__in=fruit.basket.all())

A more elegant (and possibly more performant) way could be tried as follows composing a query using Q objects (building on Dave Webb's answer to another question ):

queries = [Q(id__in=fruit.basket.all()) for fruit in fruits]

query = Q()

# AND the Q object with the ones in the list
for item in queries:
    query &= item

baskets = Basket.objects.filter(query)

Answer 2

By printing the raw SQL of the Q & operations, I've found the reason why Q is working as this.

from django.db import connection
print connection.queries
u'SELECT DISTINCT "market_basket"."id", "market_basket"."weight" FROM "market_basket" INNER JOIN "market_fruit" ON ("market_basket"."id" = "market_fruit"."inbasket_id") INNER JOIN "market_fruitname" ON ("market_fruit"."ofkind_id" = "market_fruitname"."id") WHERE ("market_fruitname"."name" IN (\'apple\') AND "market_fruitname"."name" IN (\'banana\')) LIMIT 21'

the key problem is WHERE clause will not be satisfied in a single condition when queries is used in a single filter. It is actually looking for a fruitname is both in ['apple'] and ['banana'] which is not possible. What is needed is to find (those fruits that have fruitname of 'apple') or (those fruits that have fruitname of 'banana')

Currently the only viable solution is to chain filters.

Answer 3

To get a Basket with all available Fruit instances in it, you can do something like this:

from django.db.models import Count

# first get all PKs of fruits
fruit_pk_list = Fruit.objects.value_list('id', flat=True)

# Then get filter the basket with all fruits using annotate and Count
baskets = Basket.objects.annotate(
    num_fruit=Count('fruit')).filter(num_fruit=len(fruit_pk_list))